In swivel chairs having a seat and a main screw secured to the bottom of the seat and rotatably mounted on a supporting base, it is considered essential to prevent "piano stooling," namely, to keep the vertical level of the seat constant while the seat is being rotated. It has been common practice to provide a nut assembly on the screw which will normally rotate with the screw, keeping the seat level constant, but which can be manually rotated relative to the screw for adjustment of the seat height. In the past, a keyway has been provided on the screw and a washer, having an inwardly facing key interfitting in the screw keyway, has been mounted on the screw. A nut has been threadably mounted on the screw above the washer. When the seat and the screw turn, the washer also turns, and since the nut comes in contact only with the washer and the screw, it rotates identically with them. To adjust the height of the seat it has been necessary to grasp the nut and manually rotate it on the screw. Thus, in commercially successful swivel chairs of this type, a keywayed main screw has been essential.
The keywayed main screws utilized in the prior art devices have often been subject to failure because of stresses induced in the area of the keyway. Increasing the size of the screw to accommodate these stresses has resulted in increased production costs. It has also been necessary to position the screw so that the keyway faces to the side of the chair to avoid weakening the screw in a fore and aft direction, where the major forces in a swivel chair of this type are encounterd. This introduces additional expense into the assembly cost of the chair.
It is an object of the present invention, therefore, to provide an improved swivel chair requiring no keyway in the main screw and which is therefore advantageous from the standpoint of strength, and is also economical to manufacture because it eliminates machining costs for forming the keyway and additional assembly costs incident to orienting the keyway relative to the seat.